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1. Leg 195 Summary1 Salisbury, M.H., Shinohara, M., Richter, C., et al., 2002 Proceedings of the Ocean Drilling Program, Initial Reports Volume 195 1. LEG 195 SUMMARY1 Shipboard Scientific Party2 INTRODUCTION Ocean Drilling Program Leg 195 had three distinct objectives. The first segment of the leg was devoted to coring and setting a long-term geochemical observatory at the summit of South Chamorro Seamount (Site 1200), which is a serpentine mud volcano on the forearc of the Mariana subduction system (Fig. F1). The second segment was devoted F1. Location map showing sites to coring and casing a hole in the Philippine Sea abyssal seafloor (Site drilled during Leg 195, p. 33. 30° 1201) and the installation of broadband seismometers for a long-term N Keelung 1202 subseafloor borehole observatory. During the third segment, an array of 25° Altitude/ Depth (m) 4000 advanced piston corer/extended core barrel holes was cored at Site 1202 20° 2000 1201 0 2000 under the Kuroshio Current in the Okinawa Trough off the island of ° 15 1200 4000 km Guam 6000 0 200 400 8000 Taiwan. ° 10 ° ° ° ° ° ° ° ° ° The drilling and observatory installation program at South 110 E 115 120 125 130 135 140 145 150 Chamorro Seamount was designed to (1) examine the processes of mass transport and geochemical cycling in the subduction zones and forearcs of nonaccretionary convergent margins; (2) ascertain the spatial vari- ability of slab-related fluids in the forearc environment as a means of tracing dehydration, decarbonation, and water-rock reactions in sub- duction and suprasubduction zone environments; (3) study the meta- morphic and tectonic history of nonaccretionary forearc regions; (4) in- vestigate the physical properties of the subduction zone as controls over dehydration reactions and seismicity; and (5) investigate biological ac- tivity associated with subduction zone material from great depth. The seismic observatory in the Philippine Sea is an important com- ponent of the International Ocean Network seismometer net. By filling a large gap in the global seismic station grid, the observatory will help 1Examples of how to reference the increase the resolution of global tomographic studies, which have revo- whole or part of this volume. lutionized our understanding of mantle dynamics and structure. More- 2 Shipboard Scientific Party over, the observatory will allow more precise study of the seismic struc- addresses. ture of the crust and upper mantle of the Philippine plate, as well as Ms 195IR-101 SHIPBOARD SCIENTIFIC PARTY CHAPTER 1, LEG 195 SUMMARY 2 better resolution of earthquake locations and mechanisms in the north- west Pacific subduction zone. Drilling at Site 1201 was also designed to provide more precise base- ment age constraints for models of backarc spreading in the Philippine Sea as well as high-quality sediment sections that could be used to re- construct the history of microplate motion, climate change, eolian transport, and arc volcanism in the region. Drilling at Site 1202 was designed to obtain a high-resolution sedi- ment record under the Kuroshio Current to study global climate change, sea level fluctuation, local tectonic development, and terrestrial environmental changes in East Asia over the past 2 m.y. The Okinawa Trough is one of the few locations in the Pacific where the seafloor un- der the Kuroshio Current lies above the carbonate compensation depth, allowing the calcareous microfossil record to be preserved, and it is the only location with a high sedimentation rate, allowing high-resolution studies. SITE 1200: SERPENTINE MUD VOLCANO GEOCHEMICAL OBSERVATORY South Chamorro Seamount, Mariana Forearc Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity within subduction zones. The study of input into a convergent plate margin by sampling the downgoing plate provides the geochemical reference necessary to learn what factors influence the production of suprasubduction zone crust and mantle in these environments. The study of the output in terms of magma and volatiles in volcanic arcs and backarc basin set- tings constrains processes at work deep in the subduction zone, but these studies are incomplete without an understanding of the through- put, the nature of geochemical cycling that takes place between the time the subducting plate enters the trench and the time it reaches the zone of magma genesis beneath the arc. Tectonically induced circula- tion of fluids at convergent margins is a critical element in the under- standing of chemical transport and cycling within convergent plate margins and, ultimately, in understanding global mass balance (e.g., COSOD II, 1987; Langseth et al., 1988; Kulm and Suess, 1990; Langseth and Moore, 1990; Martin et al., 1991). In the shallow to intermediate suprasubduction zone region, dehydration reactions release pore fluids from bound volatiles in oceanic sediments and basalts of the down- going plate (Fryer and Fryer, 1987; Peacock, 1987, 1990; Mottl, 1992; Liu et al., 1996). Fluid production and transport affect the thermal re- gime of the convergent margin, metamorphism in the suprasubduction zone region, diagenesis in forearc sediments, biological activity in the region, and, ultimately, the composition of arc and backarc magmas. Furthermore, these fluids, their metamorphic effects, and the tempera- ture and pressure conditions in the contact region between the plates (the décollement) affect the physical properties of the subduction zone, where most major earthquakes occur. The discovery of Earth’s deep biosphere is recognized as one of the most outstanding breakthroughs in the biological sciences. The extent of this biosphere is currently unknown, but we are becoming increas- ingly aware that life has persisted in environments ranging from active hydrothermal systems on mid-ocean ridges to deep ocean sediments, SHIPBOARD SCIENTIFIC PARTY CHAPTER 1, LEG 195 SUMMARY 3 but so far no detailed investigations have been made of the potential for interaction of the deep biosphere with processes active in conver- gent plate margins. Determining unequivocally the composition of slab-derived fluids and their influences over the physical properties of the subduction zone, biological activity, or geochemical cycling in convergent margins requires direct sampling of the décollement region. To date, studies of décollement materials, mass fluxes, and geochemical interchanges have been based almost exclusively on data from drill cores taken in accre- tionary convergent margins (e.g., Kastner et al., 1993; Carson and West- brook, 1995; Maltman et al., 1997). Large wedges of accreted sediment bury the underlying crystalline basement, making it inaccessible to drilling, and the wedges interact with slab-derived fluids, altering the original slab signal. The dehydration reactions and metamorphic inter- changes in intermediate and deeper parts of the décollements have not been studied in these margins. By contrast, nonaccretionary convergent margins permit direct access to the crystalline basement and produce a more pristine slab-fluid signature for two reasons: the fluids do not suf- fer interaction with a thick accretionary sediment wedge, and they pass through fault zones that have already experienced water-rock interac- tions, thus minimizing interaction with subsequently escaping fluids. Regardless of the type of margin studied, the deeper décollement region is directly inaccessible with current or even foreseeable ocean drilling technologies. A locality is needed where some natural process brings materials from great depths directly to the surface. The Mariana conver- gent margin provides precisely the sort of environment needed. Geologic Setting The Mariana convergent plate margin system is nonaccretionary, and the forearc between the trench and the arc is pervasively faulted. It con- F2. Bathymetric map of the south- tains numerous large (30 km diameter and 2 km high) mud volcanoes ern Mariana forearc, p. 34. (Fryer and Fryer, 1987; Fryer, 1992, 1996) (Fig. F2). The mud volcanoes 144°E 146° 148° ° 20 Conical Seamount N 0 0 0 00 are composed principally of unconsolidated flows of serpentine mud 4 0 2 Pacman Seamount 0 0 0 4000 4 2 0 with clasts consisting predominantly of serpentinized mantle peridotite 0 0 Pagan Big Blue Seamount (Fryer, Pearce, Stokking, et al., 1990). Some have also brought up blue- 18° 8 0 0 0 4000 4 0 0 2000 schist materials (Maekawa et al., 1995; Fryer and Todd, 1999). Faulting 0 8000 Turquoise Seamount 4000 of the forearc to great depth produces fault gouge that when mixed 0 Celestial Seamount 00 00 0 2 2 16° Peacock Seamount with slab-derived fluids generates a thick gravitationally unstable slurry 6000 Blue Moon Seamount 4 of mud and rock that rises in conduits along the fault plane to the sea- 000 Saipan 2 0 0 0 0 0 0 floor (Fig. F3) (Fryer, 1992, 1996). These mud volcanoes are our most di- 4 ° 14 North Chamorro Seamount Guam rect route to the décollement and, episodically, through protrusion 8000 0 6000 0 0 0 00 2 6 events, open a window that provides a view of processes and conditions 4000 8 000 South Chamorro Seamount 6000 at depths as deep as 35 km beneath the forearc. 12° Prior to Leg 195, only one other active serpentine mud volcano 100 km (Conical Seamount) (see Fig. F2) had ever been sampled by drilling (Fryer, Pearce, Stokking, et al., 1990). Little was then known of either F3. Schematic cross section the processes that formed such seamounts, their distribution, and their through the Mariana system, relation to
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